The present invention is related to an electrophotographic image reproduction system, such as a printer or a copier, wherein a latent image is formed on a photosensitive member by image-wise exposure to light using a light exposure unit based on light emitting diodes (LED) recording heads.
In a typical electrophotographic image reproduction process, first a latent charge image is formed on a pre-charged photosensitive member by image-wise exposure to light using a light exposure unit. This latent image is subsequently made visible on the photosensitive member with charged toner particles. Examples of a photosensitive member are a photoconductive drum or belt. The developed image is transferred directly or via one or more intermediate transfer members to a receptor material, where it may be fixed simultaneously or subsequently. The receptor material can be in web- or sheet-form. To generate multi-colour images, a multiplicity of latent images each of a separate colour are formed on an equal number of photosensitive members and transferred in register to the receptor material or to an intermediate transfer member to create a registered multi-color image. Depending on the configuration""s ability to print on a single side (simplex) or on both sides of the receptor material (duplex), at least four light exposure units are used in a simplex configuration and at least eight light exposure units are used in a duplex configuration. Alternately, a multi-colour image can be formed on a side of a receptor material using a single exposure unit and a single photosensitive member per side by subsequently forming latent images each of a separate colour on the photosensitive member and transferring them directly or via one or more intermediate transfer members to the receptor material.
In conventional electrophotographic systems often use is made of light exposure units including LED arrays, to generate the light, combined with an optical system to focus the light on the photosensitive member. Accurately focusing the images generated by the LEDs on corresponding points of the photosensitive surface is one of the major factors determining the image quality in such systems. As e.g. disclosed in EP629507, a LED array is typically composed of a number of LED modules, each module comprising a fixed number of LED""s. These LED modules are attached to a common carrier and connected, e.g. by means of wire bonding, to adjacently attached driver modules to thereby form a linear array of LED modules and driver modules positioned perpendicular to the propagation direction of the photosensitive member. Usually the carrier also acts as a heat sink. The light generated by the light-emitting diodes, LED""s, is accurately focused on the photosensitive member by means of a selfoc lens array being adequately positioned between the LED""s and the photosensitive member. The selfoc lens array, SLA, is composed of two linear arrays of cylindrical lenses with a parabolic refractive index distribution, each lens having equal dimensions and optical properties. The lenses are aligned between two plates, while the space in-between the lenses is filled up with silicone to fix the lenses and to prevent crosstalk. When the SLA is correctly positioned, the images of all the LED""s on the LED array are focused on the surface of the photosensitive member to form a line across the photosensitive surface perpendicular to the propagation direction of said photosensitive member. This solution works fine up to resolutions of 600 dpi. However, at higher resolutions, i.e. typically 900 dpi and above, the currently commercially available SLA""s are known to give unsatisfactory results with respect to image quality, particularly sharpness and efficiency, due to the limited optical quality of the individual lenses within the SLA. Moreover, the large non-uniformity of the lenses within the SLA, which could be corrected for satisfactory at 600 dpi, becomes problematic at higher resolutions as due to the reduced spot size the image quality is more sensitive for local discontinuities in the optical system.
U.S. Pat. No. 5,260,718 (Rommelmann, Xerox) discloses a printer with staggered image bars in optical alignment with an optical system. The modulated outputs of the image bars are transmitted as focused lines on the photoreceptor. This is enabled by tilting the optical system at angles typically between 15 and 40 degrees. The optical system is preferably a linear gradient lens array. A conventional lens system, i.e. an array of discrete lenses, would not work because it is nearly impossible to mount such lenses at the required angles in a reproducible way and moreover, this would produce unacceptable image degradation at the photoreceptor.
It is an object of the invention to provide an image reproduction system having a light exposure unit based on discrete objectives which gives satisfactory results at high resolutions, especially at 600 dpi or above.
We have discovered that this objective and other useful benefits can be achieved when the system comprises a specified configuration of a staggered plurality of linear LED arrays and a staggered plurality of discrete objectives associated therewith to expose the photosensitive member.
In an aspect of the invention an image reproduction system, including e.g. printing and copying systems, is disclosed comprising:
a photosensitive member having an outer surface;
a staggered plurality of linear LED arrays, said LED arrays being staggered such that each LED array is spaced from an imaginary plane perpendicular to the process direction of said image reproduction system;
a staggered plurality of discrete objectives, for focussing the outputs of said LED arrays on said outer surface of said photosensitive member, each of said objectives being associated with a LED array, said objectives being oriented substantially parallel to said outer surface and positioned between said photosensitive member and said staggered plurality of linear LED arrays such that the distance from each objective to said imaginary plane is smaller than the distance from its associated LED array to said imaginary plane. The objectives may be composed of glass, quartz or other transparent materials, including polymers. Preferably the distance from each objective to said imaginary plane is 40% to 60% of the distance from its associated LED array to said imaginary plane to ensure that the outputs of the respective LED arrays are projected on the outer surface of the photosensitive member within neighbouring lines. In an embodiment of the invention, the objectives are positioned such that the outputs of the respective LED arrays are projected on the outer surface of the photosensitive member on a single common line.
In another embodiment of the invention, in order to prevent crosstalk, the image reproduction system further comprises an opaque screen being mounted between said staggered plurality of discrete objectives and said outer surface of said photosensitive member; said opaque screen being provided with a slit through which the focussed outputs of said LED arrays are projected on the outer surface of said photosensitive member. The opaque screen is preferably composed of an anti-reflective material or covered with an anti-reflective coating. In an alternative embodiment, a plurality of non-reflective opaque screens is positioned between the staggered plurality of linear LED arrays and the staggered plurality of objectives, each of said screens being positioned both between two neighbouring LED arrays and their associated objectives.
In another embodiment of the invention the length of each of the LED arrays is chosen such that when put on one line, adjacent LED arrays are partially overlapping. The overlapping or partially overlapping LED""s can be individually controlled to thereby avoid discontinuous joints of projected line fragments after optically and/or electronically stitching to generate a projected image line, each of said line fragments being generated by its associated LED array. Particularly one may opt to individually modulate, or extinguish or light such overlapping or partially overlapping LED""s.